Various wireless access technologies have been proposed or implemented to enable mobile stations to perform communications with other mobile stations or with wired terminals coupled to wired networks. Examples of wireless access technologies include GSM (Global System for Mobile communications) and UMTS (Universal Mobile Telecommunications System) technologies, defined by the Third Generation Partnership Project (3GPP); and CDMA 2000 (Code Division Multiple Access 2000) technologies, defined by 3GPP2.
As part of the continuing evolution of wireless access technologies to improve spectral efficiency, to improve services, to lower costs, and so forth, new standards have been proposed. One such new standard is the Long Term Evolution (LTE) standard from 3GPP, which seeks to enhance the UMTS wireless network.
According to LTE, rank adaptation is afforded for channel-dependent precoding (closed-loop) MIMO (multiple input, multiple output) wireless transmission using information that is fed back from the mobile station to a base station. MIMO refers to the use of multiple antennas at the transmit side and/or the receive side. Rank adaptation refers to selecting from among plural ranks, such as rank 1 and rank 2, for wireless communications between the mobile station and the base station. “Rank 1” refers to use of just a single layer for the wireless channel that communicates data between the base station and mobile station. With such single-layer communications, the same signal is emitted from each of the transmit antennas (such that redundancy is provided) with appropriate phase (and sometimes gain) coding such that signal power is maximized at the receiver input. “Rank 2” indicates that a particular wireless channel used to communicate data between the base station and the mobile station is able to use two layers that employ multiple spatial beams along multiple paths in a cell sector. With rank 2 communications, independent data can be sent in multiple beams to increase throughput of the data in the wireless communication between the mobile station and the base station. Other ranks are also possible, such as rank 4 and so forth.
The information fed back by the mobile station as part of channel-dependent preceding (closed loop MIMO) to a base station includes an index to identify a codeword used for coding data to be sent on the downlink wireless channel from the base station to the mobile station. Such an index is referred to as a preceding matrix index (PMI) in the LTE context. Different indexes are used to select different codewords. The feedback information also includes the rank to be used as determined by the mobile station. Such feedback of rank information enables rank adaptation, where one of multiple ranks can be selected for wireless communications. The preceding (rank and codeword) to be applied to downlink data to be transmitted by the base station is then determined from the rank and PMI information. By applying preceding (including rank adaptation) based on feedback information from the mobile station, throughput and reliability of wireless data communications within a cell or cell sector can be improved.
However, conventional preceding for closed-loop MIMO transmission is effective only for mobile stations that are relatively slow moving such that the information (in particular, the PMI information) fed back by the mobile station to the base station is not out-of-date when the preceding is applied by the base station. As the mobile station's velocity increases, preceding used for closed-loop MIMO transmission may not be beneficial.